Peanut Butter Cup Cookie Bars

0 Preheat the oven to 325° Spray a 9"x9" baking pan with nonstick cooking spray. (You can use an 8"x8" baking pan for thicker bars if you prefer.)
1 In a large bowl, whisk together the flour, baking soda, and salt until evenly combined. Set aside.
2 In a standing mixer, beat together butter, both sugars, and peanut butter until light and creamy, 3 to 4 minutes.
3 Add eggs one at a time, mixing on low between each addition. Add vanilla and mix on low until fully blended,  about 1 minute.
4 Add dry ingredients and mix on low just until combined.
5 Stir in peanut butter cups.
6 Pour batter into prepared baking pan. Bake until the middle is just set, 26 to 28 minutes (note: add 4 t0 6 minutes if using an 8"x8" pan).
7 Allow bars to cool completely in pan. 
8 Slice and serve. Bars can be stored in a sealed container at room temperature for up to 3 days.